The invention relates to a local area network operating in the asynchronous transfer mode (ATM), more particularly, a one-way or two-way ring network, comprising a plurality of stations and network interfaces (ATM transceivers) assigned to these stations and/or further networks comprising further stations, which network interfaces have each a send and a receive ring connection, the exchange of information being effected digitally via ATM cells in whose headers are stored address data, more particularly, trunk identifiers (VPI virtual path identifiers) and link identifiers (VCI virtual channel identifiers). In addition, the invention relates to an ATM cell suitable for use in such a network and a network interface for such a local area network and, finally, a stackable ATM unit comprising a plurality of network interfaces for constructing a switching point (switch).
In a local area network operating in the asynchronous transfer mode (ATM), fixed-length blocks are formed from the information to be transmitted (messages). An additional control indication bit completes each block form to a transmission unit called an ATM cell. An ATM cell is thus formed by a header and a payload which contains the information to be transmitted.
The ATM technique is used in different switching services such as data transmission services, speech transmission services, video services (for example, video conferences, video data bases), or when multimedia information in which speech, data and video information is combined is to be transmitted. An ATM network may then be a private network as well as a public network. A public ATM network is, for example, the B-ISDN network (broadband integrated services digital network). Private or public ATM networks may be connected based on the predefined ATM standard.
With ATM, cells are uninterruptedly transmitted on each transmission section. If no payload is to be sent at a certain time, specially marked idle cells are sent. In this manner, the ATM payload cells are transmitted independently of each other, because the distances in time on the line between neighboring payload cells are generally different. This type of transmission is therefore referenced asynchronous. Thus ATM represents an asynchronous transmission of payload cells of fixed length.
A distinction is made in payload cells between user cells containing, for example, messages or data of the user of a previously set-up connection in their information field, and signaling cells containing overhead bytes in their information field, for example, for the control unit of a network interface.
In ATM networks, the available transmission capacity (bandwidth) of a broadband channel is subdivided into so-called virtual paths. A virtual path represents in its turn a group of virtual channels. When a virtual channel is set up between a transmitter and a receiver, first the suitable virtual path is selected. Subsequently, a part of the bandwidth is occupied as a virtual channel of this path. For the identification of the virtual channel is used the identifier VCI (virtual channel identifier). For the identification of the paths is used the identifier VPI (virtual path identifier). This allows each transmitted cell to be allocated unambiguously because the combination VPIandVCI (virtual path and virtual channel identifiers) represents the logic identifier for the defined connection.
In addition, a distinction is made between a unicast connection set up between a single user of a first station and a user of a second station. When the connection is set up between a first user and a plurality of further users, the connection is referenced a multicast connection. When there are an arbitrary number of further users with such a multicast connection, this is called a broadcast connection.
From EP 0 614 296 A2 is known a local area ATM network of the type defined in the opening paragraph in which both user cells and signaling cells are processed as payload cells. Each network interface is then assigned a station. The user cells then contain the actual information, for example, messages or data of the user of this connection. When a virtual link is used, for example, for transmitting speech signals (telephone call), the information field of the user cell contains speech data. On the other hand, a signaling cell contains in its information field overhead bytes which are necessary for example, for coordination of the switching function in the ring, and more particularly, for setting up a connection or clearing the connection after the information has been transmitted. On the other hand, signaling cells are also used for transmitting overhead bytes between a station and the network. In such a case, the signaling cell has a standardized structure prescribed by UNI/NNI.
In such a known network, the connection is set up in the following manner: in the case of a connection request from a transmitter station to at least one receiver station, first a signaling cell is generated containing a query about the state of the receiver station. The at least one receiver station sends at least one signaling cell in response to the query which signalling cell contains the information about its status. Upon reception of the status information, the transmitter station forms at least one signaling cell for setting up the connection if the status of the at least one receiver station allows a connection. Subsequently, the connection is set up and the information is transmitted. The connection is cleared in reverse order.
From U.S. Pat. No. 5,600,795 is known a local area network operating in the asynchronous transfer mode (ATM), in which a connection is set up in similar manner in that first a control arrangement assigned to a transmitter station receives a signaling cell coming from the transmitter station which cell contains a realizable request for a unicast or multi-cast/broadcast connection of a user of the transmitter station. Subsequently, the signaling cell is sent to a user of at least one receiver station while the signaling cell contains in its payload at least one data about the user, the address of the transmitter station and a user-related identification for the return link. Upon reception of the signaling cell originating from the transmitter station, a control arrangement assigned to the receiver station generates a signaling cell to be sent to the transmitter station, which signaling cell contains in its payload at least an indication about the address of the assigned receiver station and a user-related identifier for the down link. Also in this network, each station has its own assigned network interface.
From xe2x80x9cTelekommunikation aktuell, ATM Anwendungen, Multimediakommunikation xc3xcber Datenautobahnen, VDE Verlag GmbH 1995xe2x80x9d, pp. 11-16 is known the basic structure of ATM cells according to the prescribed ATM standard. Hence, the ATM cell comprises a payload part having a length of 48 bytes and a header having a length of 5 bytes. The header is then structured differently depending on whether it is an ATM cell which is transmitted via an interface between a station and the network (UNI, user network interface) or between two different networks (NNI, network network interface).
When a UNI payload cell is concerned, the first 4 bits of the header form the GFC function (generic flow control). This makes it possible to avoid overload situations in the network due to a transgression of predefined bandwidths. Subsequently, a bit sequence of 8 bits VPI and 16 bits VCI follows for defining the virtual path and virtual channel, respectively. The next information of the header cell relates to the type of cell (PT, payload type) with which a distinction is made in this state of the art between user cells and signaling cells. Further information is given by the cell loss priority (CLP) and the header error control (HEC). The above structures PT, CLP, HEC are identical in UNI/NNI cells. In contrast, the NNI cell distinguishes itself from the UNI cell in that in lieu of the GFC structure occurring in the UNI cell, the structure for the path identifier (VPI) is extended from 8 to 12 bits.
It is an object of the invention to further develop a network or network interface respectively, of the type defined in the opening paragraph, and an ATM cell suitable for being processed in such a network, in that the communication in the ring is simplified without the standard determined outside the ring being violated.
This object is achieved with a network or network interface respectively, of the type defined in the opening paragraph, in that the at least one network interface is assigned converter means for at least partly and reversibly converting data bytes available in the header of the ATM cell into an identifier specific of a network.
In an ATM cell to be used in a network of the type defined in the opening paragraph, the solution according to the invention is that a multiplexer/demultiplexer arrangement is assigned to such an ATM cell which arrangement forms a port extender via which the data transfer takes place to a plurality of stations assigned to connection ports.
The invention versus the state of the art is characteristic in that the conversion of the data bytes available in the header bits of the ATM cell into the identifier specific of the network may provide a much more flexible data transfer in the network. The solution according to the invention is based on the approach that a standardization according to the UNI/NNI standard is to be maintained only where this is necessary on the basis of conditions with respect to the network environment.
As opposed to this, the prescribed standard need not be adhered to in the network, thus seen from the network on this side of the interfaces to the stations. A header format specific of the system had rather be used here. The two said formats are distinguished in the meaning of the header bits, but not in the length of the header. When entering the network, the header corresponding to the UNI/NNI standard is thus translated according to the invention into the header bits specific of the network. Conversely, when the cell leaves the network, its header is converted back into the UNI/NNI format. The selection of the header bits used for the identifier specific of the ring may then be free.
As a result of the fact that preferably the converter means include a header translation table updated during the call set-up or call cleardown phase, so that at least part of the header not occupied by the standardized VPI and VCI bit sequence has been overwritten with the network-specific identifier, there is a simple adaptation possibility. The header translation table is then defined during the signaling (call set-up) so that, on the one hand, the VPI and VCI bit sequence is maintained for the virtual channel identifier and that, on the other hand, the desired network-specific information is taken into account in the identifier. A first zone of the identifier may preferably be used for distinguishing unicast (single), multicast (plural) and broadcast (multiple) connections. A further zone called zone II in the following, of the network-specific identifier is used for indicating the destination of the ATM cell i.e. for determining the network interface to which the destination station is connected.
Finally, also indications about the priority and the property of a cell (control cell) may be stored in the network-specific header format.
In the unicast mode, the name of the network interface to which the desired destination station is connected is contained in the further zone of the network-specific identifier.
In contrast, in the multicast or broadcast mode, the information stored in the further zone of the network-specific identifier corresponds to the address of the sending network interface (Port-ID=SENDER ID). The ATM cell is thus transferred in the network from interface to interface until it finally arrives at its starting point, i.e. at the send network interface and is erased there. In the broadcast mode it is not necessary to make any further distinction, because the ATM cell is intended for each network interface. As opposed to this, there is to be determined beforehand in the multicast mode whether the ATM cell is meant for the respective network interface, as there is a station there belonging to the connection, or whether the cell is transferred to the output of the network interface without being switched to the station. For this purpose, a small broadcast/multicast table is used which is defined by the so-termed call handler during the signaling phase based on the stations desired for the connection. This small broadcast/multicast table on the station output is necessary, because the cells belonging to a certain broadcast/multicast connection use a global i.e. substantially identical VPIandVCI pattern. On the other hand, according to the UNI/NNI standard, different connection users also have different VPIandVCI values. The broadcast/multicast table therefore adapts the global VPIandVCI pattern of a connection in the ring network to user-specific VPIandVCI patterns according to the UNI/NNI standard. This adaptation takes place during the signaling phase. The effect of this broadcast/multicast table is that an erase notifier for erasing the cell from this table is read out for the case where in the multicast mode a cell is concerned that does not belong to the station belonging to the network interface.
A particular embodiment of the solution according to the invention comprises that at least one network interface includes a multiplexer/demultiplexer via which at least two stations are connected in common to the network interface. Accordingly, henceforth a plurality of stations corresponding to the multiplicity of the multiplexer or demultiplexer respectively, which stations may be operated from the same network interface, are assigned to each network interface. Thus the network interface has the feature of a so-called port extender which includes a fixed number of connection ports for the individual stations. The cells arriving at a comparably slower speed from the stations are transformed by the multiplex process to the high speed link to the send ring connections of the network interface. Conversely, the cells coming from the network interfaces and arriving at the different connection ports for the stations are distributed in the sense of a demultiplexing. This multiplex and demultiplex function of the port extender may be realized without the necessity of changing the above-mentioned basic structure of the network interface arrangement. Only measures need to be taken for putting complementary information in the further sub-structure of the network-specific identifier, on the basis of which complementary information the fact is taken into account that henceforth not only one but a plurality of stations are connected to each network interface via the port extender. When applying this solution and using of the port extender, it is particularly advantageous that the UNI/NNI standard is available at the input or output respectively, of the port extender, that is to say, is shifted thereto at the network interface.
For this purpose, in the case of the unicast mode there is provided that the further zone II of the network-specific identifier comprises two bit sections x, y, the one bit section x being used for identifying the respective access port connected to the multiplexer/demultiplexer and the other bit section y for identifying the network interface for which the ATM cell is intended. In contrast to the above-defined basic system without a port extender, henceforth the first bit section x is completed and the second bit section y is shortened. This refers to a further small broadcast/multicast table from which the call handler receives the information about which access ports are addressed to the respective network interface in what manner. When in the unicast mode the transmitter station selects a certain receiver station, the call handler is supplied with the coding information for the bit section x via the allocation known to it. The data bits are then stuffed so that in the x structure the information is stored for the respective access port and in the y structure the identification of the network interface including this access port.
In the case of a multicast/broadcast mode, the further zone (II) of the network-specific identifier has the indication about the sending network interface (SENDER-ID), as in the basic system without a port extender. Here too the ATM cell thus runs from interface to interface until it finally arrives back at its point of departure.
Also in the broadcast mode there is no substantial difference when the port extender is used.
In the multicast mode, however, there is provided that each network interface taking part in the connection has an allocation pattern defined during a call set-up, which pattern is used for selecting the access port taking part in the connection in this network interface, with which port the further zone II of the network-specific identifier is overwritten in the network interface. This measure takes account of the fact that in the multicast mode, on the one hand, the information is to be received whether the respective network interface actually has access ports serving the stations belonging to the connection and, if so, in addition, the further information which of the stations are involved in the connection. The first information is then known to the call handler via the system structure. The allocation pattern contained in the second information is determined during the signaling phase in accordance with the stations taking part in the connection and is then used at a later time as a distribution algorithm for the incoming cells in the output ports of the associated port extenders.
A particular way in which the solution according to the invention is used is that a network interface is set up which forms a local blocking-free Nxc3x97N switching point when the data rate of the connection between port extender and transceiver exceeds the sum data rate of the individual port extender inputs or outputs, respectively.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.